The present invention relates to waveguide-to-coaxial line transition apparatus, and more particularly to a convertor apparatus for converting to or from a TM.sub.01 waveguide mode in circular waveguide from or to a coaxial transmission line, respectively.
The TM.sub.01 mode has unique characteristics which are often utilized in circular waveguide components. To measure such components it is necessary to be able to sense and/or generate the TM.sub.01 mode. However, the coaxial configuration is the commonly used mode on most test equipment, and a mode convertor to the TM.sub.01 mode is required. Mode conversion is also sometimes desired within a component, and this approach is directly applicable.
There are two known approaches for conversion between coaxial line and the TM.sub.01 circular waveguide mode. The first involves inserting the center conductor of a coax line into the endplate of a circular waveguide, thus acting as an excitation probe. "Input Impedance of a Coaxial Line Probe Feeding a Circular Waveguide in the TM.sub.01 Mode," R. H. MacPhie et al., IEEE Tran. Microwave Theory Tech., Vol. MTT-38, pp 334-337, March 1990. The probe is concentric and aligned with the waveguide axis. While this configuration is simple, it is narrow band and requires tuning of the probe length to get a match. Moreover, the operating bandwidth is limited to approximately 10% by the excitation of the higher order "TM.sub.01 -coax" mode.
The second approach first adapts the coaxial line to rectangular waveguide, splits the rectangular waveguide into two arms, and then couples to the circular waveguide with two symmetrically mounted, in-phase apertures. This configuration is large relative to the circular waveguide, is not well matched and has an instantaneous bandwidth of only about 10%, being limited on the upper end by excitation of the circular TE.sub.21 mode.
It is therefore an object of this invention to provide a well-matched, wideband device for mode conversion between the coaxial (TEM) and the TM.sub.01 circular waveguide modes.
A further object is to provide a mode convertor for conversion between the coaxial mode and circular waveguide mode which maintains mode purity over the entire TM.sub.01 band, up to the frequency at which the TM.sub.02 mode begins.
Yet another object is to provide a device for conversion between the coaxial mode and circular waveguide mode which is relatively small and simple to fabricate.
Yet another object is to provide a device for conversion between the coaxial mode and circular waveguide mode which allows for maximum power transfer between the modes.